Most commonly, an ostium is defined as any kind of small opening in the body. These openings can be external, such as pores, or internal, such as the chambers of the heart. It is also a name for a small hole in a sponge.
There are many kinds of these openings in the human body. They occur in the heart, nose, fallopian tubes and skull. They can carry blood or water inside or outside the body, or serve as an opening for veins to pass through.
The maxillary sinus, a sinus chamber located under the eye, has an ostium to drain mucus secreted by the sinuses. The maxillary sinus is lined with cilia, which help move the mucus toward the opening. When a human being experiences a cold or allergies, they will not be able to drain the mucus fast enough, resulting in sinus pain.
The foramen ovale is a type of bony ostium. It forms one of the larger holes in the base of the skull, and provides an opening through which nerves and veins connect from the brain to the rest of the body. The ostium of the fallopian tube is located in the female reproductive system and connects the uterine tube to the abdominal cavity. During ovulation, an ova, or egg, passes through this opening to reach the fallopian tube. From there, it travels to the uterus to possibly be fertilized.
Not all ostium are permanent. Several are formed in the human heart during embryonic development. These openings will later grow shut as the human circulatory system develops.
For example, the ostium primum is located in the heart of human embryos. During development, the heart does not have well-developed separate chambers. Later, these chambers grow closed through the development of separating walls of tissue, called septum. The original opening is sealed shut, and a second opening, called the ostium secunda, opens in another place. Failure of the ostium primum to grow closed properly is one of the most common causes of heart defects.
Non-mammalian bodies can also have ostium. Sponges are multi-cellular sea creatures that have no definite organs. Instead, they have outside pores that take in water and nutrients from outside the body. The interior of a sponge is divided into chambers for absorption. These pores take in nutrients and expel them, sometimes out of the same opening.